From Sahih Bukhari (Hadith 7203):
Narrated `Abdullah bin Dinar:I witnessed Ibn `Umar when the people gathered around `Abdul Malik. Ibn `Umar wrote: I gave the Pledge of allegiance that I will listen to and obey Allah's Slave, `Abdul Malik, Chief of the believers according to Allah's Laws and the Traditions of His Apostle as much as I can; and my sons too, give the same pledge
Reference: Bukhari 7203